Reply by Paul Pomerleau

”So you would put the washer on the side facing away from the fridge?”

Yes, a bare magnet has waves of flux permeating all around it, a washer (or any ferromagnetic medium) will focus the waves away from it, increasing its strength. Putting the magnet in a metal cup so the sides are also covered (like what you can find here) will force the waves even tighter giving you the maximum strength of a magnet.A 1” rare earth magnet in a ferromagnetic cup can easily lift 25 pounds.